The blades used in the turbine section of a gas turbine engine are susceptible to oxidation, corrosion and thermally induced damage from hot combustion gases flowing through the engine's gaspath. Turbine blades are therefore made from a temperature tolerant, corrosion resistant substrate material to which various protective coatings are applied. In addition, turbine blades usually include numerous transpiration cooling passages, each of which extends from an internal cavity to a gaspath exposed surface of the blade. Each passage has a throat in the vicinity of the cavity and a mouth that extends outwardly from the throat to the exposed surface. During engine operation a cooling medium is supplied to the internal cavity, and at least a portion of the medium flows through the cooling passages to transpiration cool the blade. As the medium discharges from the passages, it disperses over the blade surface to form a cooling film that further shields the blade from thermal distress.
Because turbine blades are expensive, a variety of refurbishment techniques have been developed to restore deteriorated or damaged blades to serviceable condition. The specific details of the various refurbishment techniques depend on the nature and extent of blade damage and deterioration. However certain procedures are almost invariably carried out during refurbishment. For example, it is customary to relieve any residual blade stresses by heating the blades to an elevated temperature for a predetermined period of time. In addition, existing protective coatings are usually removed from the blades, and nonoriginal replacement coatings are applied prior to returning the blades to service.
When a nonoriginal coating is applied to a blade having transpiration cooling passages, excess coating can accumulate in the mouth of each passage. This phenomenon is known as "coatdown" and restricts the flow capacity of the affected passages. Unless coatdown is prevented, or its flow restricting effect is reversed, the effectiveness of the transpiration cooling and film cooling will be diminished, and the blade's useful life will be reduced.
One way to prevent coatdown is to coat the blade by vapor deposition, a coating process that causes little or no coatdown. However the equipment for applying coatings by vapor deposition is expensive, and therefore it is economically unattractive to use the vapor deposition equipment to apply coatings that can be applied by more cost effective means.
One way to reverse the effects of coatdown is to erode the excess coating by propelling a high velocity, precisely focused stream of abrasive particles into the mouth of each affected passage. However the erosive treatment can be inaccurate and nonrepeatable. Therefore the effectiveness of the treatment must be assessed by verifying that a gauge pin, representative of the minimum acceptable passage dimension, is insertable into each passage. Although the erosive process is effective in restoring the flow capacity of a passage, it is also tedious and time consuming since a typical turbine blade has nearly two hundred passages, each of which must be treated and gauged individually. Moreover, the gauge pins are necessarily fragile due to the small diameters of the passages (typically on the order of 0.3 millimeters or about 0.012 inches) and occasionally break, leaving a pin fragment lodged in the passage. Extraordinary measures, such as electro-discharge machining, must often be employed to clear the fragment from the passage.
Thus, it is seen that existing methods for avoiding or reversing the effects of coatdown are unsatisfactory. Accordingly, a time efficient, cost effective and trouble free method of accommodating the coatdown phenomenon is sought.